Wave signaling system



.1929- w. A: MacDONALD 1,721,520

" WAVE SIGNALING sYs'rEI Filed June 23, 1928 INVENTOR Wm.A. Mac Donald,

ATTORNEYS Patented July 23, 1929.

UNITED STATES 1,121,520 PATENT" OFFICE.

WILLIAM A. MACDONALD, Q1 LITTLE NECK, NEW YORK, ASSIGNOR TO HAZELTINE CORPORATION, OF JERSEY CITY, NEW JERSEY, A CORPORATION OF DELAWARE.

WAVE SIGNALING SYSTEM.

Application filed June 23,

This invention is an improvement in vacuum tube radio receiving systems and pertains sperilically to receivers equipped with loop antennas and multistage tuned amplifiers with conjoint tuning control means for simultaneous operation of the sundry tuning condensers.

The present application is a continuation in part of my co-pending application Serial No. 85,615, tiled l -ebruary 3, 1926, issued August 14, 1928, as Patent No. 1,680,424.

The object oi the invention is to facilitate the attainment of mutual resonance (syntonism) between a plurality of tunable circuits (of which one is a loop antenna circuit while the others are interstage coupling circuits) adapted to cover a relatively Wide band of frequencies such as the broadcast band arranged to be operated simultaneously through the agency of a conjoint tuning control device.

It is Well recognized as a fundamental truism that in order to realize satisfactory operation of a multistage tuned amplifier With unified tuning control. it is essential that all of the tunable circuits be resonant to the same frequency at each and every setting of the tuning control device. There is more than one way in which this might be done, but according to the best opinion, which, indeed, is well founded, it is essential, or substantially so in commercial practice, that the inductances of the several circuits be equal and that the variable tuning condensers be electrically identical. That is to say, the tuning condensers should have identical capacities for each and every setting of their conjoint operating device. These conditions having been met, it is manifest that if the circuits are physically alike, mutual reso2.-.ince at all settings of the tuning control device will follow as a natural consequence. Such a result does not usually obtain, however, even when the inductances are equal and. the tuning condensers identical, unless the circuits are physically alike. This is because of the existence of capacities in the circuits extrinsic to the tuning condensers-said extrinsic capacities being unequal.

For loop-equipped broadcast receivers which have to submit to appraisal on the basis of their artistic merit as well as that of their reproducing efficiency, it is highly desirable that the loops be as compact and 1928. Serial No. 287,718.

unobtrusive as practical operating requirements will permit. From this it follows that it is decidedly preferable, from the esthetic standpoint, that the loop turns be wound close together. This, however, usu ally results in a distributed loop. capacity somewhat higher than the corresponding distributed capacity across the inductances of the other tunable circuits-which means (under the conditions predicated) that the loop circuit would be out of resonance with the other tunable circuits with which it is associated. To obtain the essential mutual resonance, there must be brought about an equalization of extrinsic capacities. How this can be done most efficiently and economically is the subject with which we are here concerned.

It can be done by increasing the minimum capacities of each of the tunable circuits other than the loop circuit or by reducing in one way or another the minimum capacity of the loop circuit. If the capacity difference is substantial as is likely to be the case, the first mentioned method is objectionable because it involves the use of padding condensers of substantial capacity, and this in turn necessitates a substantial increase in the maximum capacity of each tuning condenser in orderthat the entire frequency range may be covered-it being quite well known that the maximum capacity of a tunable circuit designed to cover the present broadcast range (550 to 1500 kilocycles) must be approximately nine times the capacity required at the highest frequency of the broadcast band.

The present invention provides a practicable economical solution of the problem under consideration in that it does not necessarily involve any increase in the size of the tuning condensers; nor does it introduce any objectionable complexity of any sort. This is accomplished by connecting the input terminals of the first radio frequency ampliiier tube across a part only of the loop turns instead of across the entire loop-the tuning condenser, however, being connected across the entire loop. Aswill be explained, this results in a decrease of effective minimum capacity of the loop circuit and this decrease may be made such as to bring about an equalization of the total effective capacity of the loop circuit with that of each of the other tunable circuits. Hence the loop circuit may be brought into mutual resonance with the other tunable circuits while retaining the most desirable loop design and, generally, without having to resort to the use of padding condensers and without having to make any initial adjustments of the individual tuning condensersboth of which expedients are deemed to be more or less objectionable or, at best, uneconomical.

An explanation will now be offered as to why and how an adequate reduction of effective capacity can be brought about in the loop circuit by simply connecting the input terminals of the first t'ube across a part only of the loop turns. This explanation will be presented in conjunction with the circuit diagram depicted in the accompanying drawing.

The dia ram represents the radio frequency end of a loop-equipped radio receiver comprising two stages of tuned radio frequency amplification followed by a detector. The two radio frequency amplifier tubes designated 1 and 2 respectively are coupled in cascade through a tuned radio frequency transformer and the latter is coupled in like manner to the detector tube 3.

There are three tunable circuits, the first of which comprises a receiving loop 4 and a variable tuning condenser 5 connected across its extreme terminals. The other two tunable circuits comprise the secondary windings 8 and 9 respectively and the associated variable tuning condensers 6 and 7. Neutralization of capacity coupling in the amplifier stages is provided for in accordance with the well known Hazeltine methodthe neutralizing condensers being designated 10 and 11 respectively.

The three tuning condensers 5, 6 and 7 are indicated, symbolically, by the dotted line 12, as being mechanically interconnected for operation through a conjoint tuning control device. Such a device is not shown but may be of any suitable form, many of which are well known.

As previously intimated, it is to be assumed that the condensers 5, G and 7 are electrically identical. That is to say that they display identical capacities or substantially' so at each and every setting of the conjoint tuning control device. This is a condition precedent which is deemed essential in the productionof radio receivers of this type on a commercial scale. Likewise it is to be assumed that the inductance of loop 4 is equal to that of each of the two second ary windings 8 and 9.

If, for the purpose of expediting this explanation, it be fictitiously assumed for the moment that the input terminals of the tube 1 are connected across the entire loop instead of only a portion thereof (as is actually the case), the total minimum capacity of the loop circuit will equal the sum of the. following: (1) the minimum capacity of condenser 5; (2) the distributed capacity of the loop; (3) the capacity of the neutralizing condenser 10; (4:) the capacity between the grid and filament of tube 1, including that between the lead-in wires and contact springs; and (5) the capacity between the input circuit leads. The total minimum capacity of each of the other tunable circuits is comprised of corresponding items; but the total minimum capacity of the loop circuit exceeds that of the others because of the larger distributed capacity of the loopa condition which, as before stated, arises from the commercial expediency of making the loop as compact and unobtrusive as practical operating requirements will permit.

As depicted in the circuit diagram, the

input electrodes of the first tube are connected across one half of the loop. Hence the extrinsic capacity (other than the distributed capacity of the loop) is manifestly in parallel with one half of the distributed capacity of the loop and in series with the other half of the distributed capacity. Obviously this results in a reduction of the total eifective minimum capacity of the loop circuit. It follows, logically, that if the distributed capacity of the loop is not too greatly in excess of that of the secondary windings 8 and 9, equalization and mutualucsonance can be brought about in this way. Actual expe-' rience has proven this to be the case and has shown that the solution of the problem here presented is, indeed, a very practical one.

In the drawing the gridlead is shown connected to the mid-point of the loop, but it is not to be supposed that this is necessarily the optimum point of connection. Of course the larger the number of loop turns between the input terminals, the greater will be the voltage developed thereacross. But it is not always desirable to develop the maximum possible input voltage at the terminals of the first tube. In fact to do so may lead to undesirable results such as instability. At any rate it is found that there is no apparent loss of sensitivity arising from the fact that the entire loop is not included between the input terminals unless the reduction of turns across the input terminals is carried to unreasonable extremes.

To those who are skilled in the art, it will or should be immediately evident that it will not do to design or select a loop haphazardly where the advantages of this invention are to be availed of. The loop design must be determined with a View to a distributed capacity which is not too much in excess of the distributed capacity of the other inductances. The limitation, however, is not so restrictive as it might seem because in actual practice the invention makes possible the provision of loops of excellent proportions Loop circuit capacities.

Micro microtarads.

Distributed capacity of loop 30 Input capacity of first tube, including socket 18 Neutralizing capacity 1 8 Input wiring 5 Minimum capacity of tuning condenserz- 15 Total minimum input circuit capacity- 76 lnterstage coupling system capacities.

Micro microtarads.

Distributed capacity of coil, (secondary winding) 10 Input capacity of tube and socket 18 Neutralizing condenser 8 Input wiring 3 Minimum, capacity of tuning condenser 15 Total 54 nately large tuning condensers in order to I enable covering the whole frequency band. In the particular case cited the maximum capacities of the tuning condensers would have had to be increased to the extent of approximately 200 micro microfarads.

Now let it be assumed in connection with the example cited that the grid is connected to the mid-point of the loop. Then the minimum capacity across the entire loop comprises the distributed capacity of the loop, namely, 30 micro microfarads and the minimum capacity of the tuning condenser which is 15 micro microfarads. The residuum of effective loop circuit capacity is then equal to one quarter of the remaining extrinsic capacity which is one quarter of the sum of the tube input capacity including the socket, together with the capacity of the neutralizing condenser and that of the input wiring. This makes a total minimum effective loop circuit capacity of 52% micro microfarads which is very close to the total minimum capacity of each interstage coupling system which was given as 54 micromicrofarads. This small difference can be compensated for by the use of padding condensers Without greatly increasing the size of the tuning condensers; or more exact equalization can be realized by placing the grid connection at a different point on the loop. Practical expediency will usually dictate which is the better course to follow. In some cases it may be found preferable to compromise by adding small padding condensers and connect a larger portion of the loop between the input electrodes of the first tube than would otherwise be practicable.

The example cited above represents a typical case but is not to be regarded as in any way definitive of the limits and scope of the invention.

I claim:

-1. A higlrfrequency wave signaling system comprising a plurality of co-operant tunable circuits in cascade, each of said tunable circuits including a variable tuning condenser and an inductance coil, said condensers being substantially electrically identical, conjoint tuning control means interconnecting said condensers whereby they are operable simultaneously, the first in order of said tunable circuits including as its inductance coil a loop antenna comprising a plurality of turns, said loop antenna having inductance substantially equal to that of the other said inductance coils and having a larger distributed capacity than that of the other said inductance coils, and a vacuum tube having its input terminals connected across a part only of the turns of said loop antenna, the arrangement being such that all of said tunable circuits are rendered substantially mutually resonant throughout their operating frequency range notwithstanding said oop antenna having a larger distributed capacity than that of the other said inductance coils.

2. A high-frequency wave signaling system comprising a tunable loop circuit including a loop antenna havin a plurality of turns, a plurality of ampliing vacuum tubes operatively associated with said loop circuit, and a tunable coupling system interconnecting said amplifyin tubes in cascade, said loop circuit and said each including a variable tuning condenser, said condensers being substantially electrically identical and mechanically interconnected for conjoint operation, said coupling system including an inductance coil having an inductance of a value substantially equal to that of said loop antenna and having a.

coupling system distributed capacity less than that of said loop antenna, one of said vacuum tubes having two of its electrodes connected across a part only of the turns of said loop antenna whereby the effective capacity of said loop circuit is made substantially equal to that of said coupling system so that the electrical characteristics of said tunable circuits of the system are substantially identical.

3. A radio receiving system comprising a plurality of tunable circuits coupled in se riatim through the medium of an interposed vacuum tube, each of said tunable circuits comprising an inductance coil and a variable -tuning condenser, said condensers being substantially electrically identical and inechani cally interconnected for conjoint tuning control, said inductance coils being of substantially equal inductance, one of said inductance coils being a loop antenna and having a larger distributed capacity than that of the other of said inductance coils, said vacuum tube having input electrodes connected across a smaller portion of said loop antenna than that across which the loop circuit tuning condenser is connected, whereby the effective capacities of the several tunable circuits are substantially equal, wherefor said tunable circuits are in substantial resonance throughout the operating range of said tuning condensers.

In testimony whereof I aflix my signature.

WILLIAM A. lthcDONALD. 

